The present invention relates generally to a method of communicating in a radio frequency identification system.
In applications for identification of persons and things, optical bar coding technology is almost universally employed. Generation of the bar code is very inexpensive but limited. One problem associated with bar codes and bar code readers is that the bar codes must be precisely aligned with the bar code reader in order to be read. Another problem with bar codes is that the bar codes may become unreadable as a result of damage due to, for example, exposure to moisture, or wear and tear from use. Radio frequency identification (xe2x80x9cRFIDxe2x80x9d) tags address some of the shortcomings of bar codes and have been proposed as a replacement for optical bar codes in at least some applications. RFID tags used in bar code applications are sometimes referred to as electronic bar codes.
Remotely powered electronic devices and related systems for powering up and receiving stored information from such devices are well known. For example, U.S. Pat. No. 4,818,855 issued to Mongeon et al., titled, Identification System, discloses a remotely powered identification device which derives power from a remote source via one of an electric field or a magnetic field and which transmits stored information back to the source via the other of the electric field or magnetic field. Remotely powered identification devices of this type are commonly referred to as RFID tags. A power source with a data collection function is known as a tag reader. A power source capable of sending data to a tag is known as a tag writer. A power source capable of bi-directional communication is known as a tag reader/writer.
An ongoing objective in the development of RFID tags and associated readers and/or writers of the general type described above has been to minimize cost and size, and to improve efficiency of operation. The simplest and least expensive RFID systems employ unidirectional communication, allowing data transfer from tag to reader only. These are commonly known as read-only systems. In read-only systems, eliminating the need for a data receiver on the tag minimizes tag cost. Typically, these tags transmit information continuously as long as they receive adequate power from the source, wherein lies the primary system limitation. The reader""s receiver is capable of reliably detecting data from only one tag at a time. If multiple tags are present within the reader""s field, they will simultaneously transmit and create mutual interference at the reader""s receiver, preventing the data from any one tag from being recovered successfully. This mutual interference condition is commonly referred to as a data collision. The terms anti-collision and collision mitigation are used to describe methods employed to prevent or minimize the impact of such data collisions at the reader.
Prior RFID systems have used the Aloha protocol for anti-collision. The Aloha protocol requires substantial bi-communication between the reader and the tags. This is undesirable from a tag complexity (cost) point of view. The Aloha protocol sorts through a population of RFID tags and assigns each tag a unique node address. This node address is subsequently used to provide collision free communication between the tags and the reader. The reader sends out a request command to all tags in the field. The tags react to the request command by selecting a random number. This random number defines the tag""s channel identification or slot number. The reader polls the tags in the field looking for a response. The reader starts by polling for slot number 0. All tags that have chosen a random number of 0 respond. If exactly one tag responds, then the reader assigns a unique node address to that tag. If more than one tag responds, a collision will occur. The reader will ignore this indecipherable response. If no tags respond, the reader moves onto the next slot. This process continues by polling for slot number 1. Again, if a single response occurs, the tag is assigned a unique node address; otherwise, the polling sequence proceeds with the reader polling for the next slot number. Upon reaching the last slot, the reader can start over by requesting tags that have not been assigned a node address to select a new random number. The entire polling process is repeated until all tags in the field have been assigned unique node addresses. At this point, the reader can select an individual tag for subsequent communication by specifying its unique node address, providing a collision-free communication channel.
The problems with the Aloha protocol are the requirement for substantial bi-directional communication and the additional circuitry on the RFID device required to perform random number generation. It is desirable that a RFID device employs a low cost, simple circuitry. The requirements for a good tag receiver, a hardware random number generator, and protocol processing circuitry can double or triple the cost of the RFID device. It is readily apparent that a simpler more efficient method is required.
A second means for anti-collision involves arbitration. In an arbitration system, collisions are allowed. The reader acknowledges received data on a bit-by-bit basis effectively selecting which tag is to be read. The arbitration process starts with the reader issuing a start signal. All powered tags respond by transmitting their first bit. The reader uses a maximum likelihood detector to select a received bit (arbitration), and then acknowledges the reception of the received bit with a transmission to the tags. The reader transmits a one acknowledgement if a one was received and a zero acknowledgement if a zero was received. All tags that transmitted the acknowledged bit remain active. Tags that transmitted a bit that do not match the acknowledged bit become inactive and remain inactive until the next start signal is received. Active tags then transmit their next bit. The arbitration process is repeated. At each loop of the cycle, non-matching tags are inactivated. Eventually a single tag is completely read. The tag that was read stops participating in the arbitration process and all inactive tags become active upon reception of a new start signal. The process continues until all tags in the field have been read.
A typical arbitration system will read the data on all tags in the powering field in at least two times the amount of time required to transmit the data from the tags to the reader. This doubling of total elapsed time for data transmission results from the overhead of the acknowledgement signal. The elapsed time is important in applications in which read time may limit throughput, such as when tagged items must be read on a moving conveyor system.
Thus, there exists a need for providing a method and apparatus that fosters efficient communication between a reader and a plurality of tags in a RFID system.